High performance communications cables are required to allow future growth in computer networking speeds and other applications which require digital electronic equipment to communicate by the rapid transfer of data. Metallic core based communication cable, in particular of the “conductor pairs” type, allow digital electronic equipment to transmit/receive data via electrical signals transmitted at various transmission frequencies.
A high performance communications cable generally must achieve a high level of performance while adhering to industry standards such as requirements set by AS/NZS 3080:2000, ISO/IEC 11801:2000, EIA/TIA 568-A:1999, or NEMA WC 66:1999 standards. For example, EIA/TIA 568-A for Category 5 cables regulates the performance of communication cable up to a transmission frequency of 100 MHz.
In addition to impedance, attenuation, and crosstalk, the EIA/TIA 568-A standard specifies dimensional constraints that must be adhered to when manufacturing high frequency communication cables.
High performance communications cables which are capable of performing at high transmission frequencies while meeting or exceeding the relevant industry standards require special consideration to reduce factors such as the degree of crosstalk. The communication cables must achieve high transmission frequencies while maintaining the integrity of the transmitted data.
Crosstalk is an important factor in evaluating data cable performance. Crosstalk represents signal energy loss or dissipation due to coupling between conductors or components of the cable. Crosstalk coupling within a cable is related, among other factors, also to the dielectric constant of the materials used in the cable.
Communications cables with cores that have groups of conductor pairs (also known as “twisted pairs”) in the same cable present the problem of crosstalk between the different groups of conductor pairs. With an increase in transmission frequency the crosstalk problem increases, and cables that were acceptable at a lower transmission frequency may be no longer adequate.
For manufacturing a communication cable, a polymeric sheath is extruded onto a plurality of such twisted pairs. In some cable designs, the polymeric sheath is extruded directly onto the twisted pair. Alternatively, the twisted pairs can first be grouped together by enclosure into a first thin sheath, e.g. by wrapping the group of twisted pairs with a polymeric tape, and then an outer sheath of polymeric material is extruded about the grouped twisted pairs. An example of such cable is sold by Pirelli Cables Australia Ltd under code L25P5.
The Applicant has however observed that such a design of cable may cause in some instances crosstalk problems in the transmitted signal. In the perception of the Applicant, these problems may be caused by an incomplete or irregular contact of an intermediate layer, disposed about the group of twisted pairs, with the outer polymeric sheath, with consequent impedance variation and crosstalk penalty caused by capacitance through the outer sheath.
The Applicant has thus observed that an improvement in the adhesion between said intermediate layer and the outer sheath of the cable could result in improved transmission properties of the cable.